Problem with (setf dialog-item-text)

[gersh@aplpy.jhuapl.edu (John R. Gersh)]

In article <199305201556.AA01913@imag.fr>  Gilles Serasset writes:
>Hello,
>
>I am defining a view with an icon and a text. The size of the view depends
>on the text:
>
>

		[Much Lisp code deleted]

>
>;;Everything works great, when I change the dialog-item-text, the size is 
>;;recalculated and the dialog-item is redisplayed properly.
>
>;;Then, I define the method for (setf dialog-item-text):
>
>(defmethod (setf dialog-item-text) ((self icon-and-text-dialog-item) text)
>  (set-dialog-item-text self text))
>
>;;It does not work, the text is change, but the size is not recalculated and the
>;;icon not redisplayed. WHY ????????
>

This is a subtle one. CLtL2, section 28.1.2.4 (p. 777), says that the
generic function for writing into a slot "takes two arguments: the new
value and the instance, in that order." If an accessor (rather than a
writer) is defined, the name of the generic function is "(setf
<slot-name>)" rather than "<slot-name>," but, presumably, the order of
its arguments is the same.

The definition above for (setf dialog-item-text) has arguments in the
order: instance, new-value, with the _first_ argument specialized on
the icon-and-text-dialog-item class. It does not agree, therefore,
with the method for (setf dialog-item-text) constructed by the
defclass for dialog-item, and inherited by icon-and-text-dialog-item,
which specializes the its _second_ argument on the class.

The defmethod above, therefore, does not replace the existing one in
the generic function (setf dialog-item-text), it adds one with
different argument specialization. [CLtL2 p.789] Consequently when the 
example calls:

	(setf (dialog-item-text di) "Salut")

it's the original method that's called, one that just sets the slot's
new value.

If the method above is defined

(defmethod (setf dialog-item-text) (text (self icon-and-text-dialog-item))
   (set-dialog-item-text self text))

then things seem to work as desired.




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